1. Field of the Invention
The present invention relates generally to an auto iris/gamma correction apparatus and an image sensing apparatus comprising such an auto iris/gamma correction apparatus, and more particularly, to an apparatus for automatically making exposure adjustment and/or gamma correction in response to a video signal obtained from an image sensor and an image sensing apparatus such as a video camera comprising such an auto iris/gamma correction apparatus.
2. Description of the Background Art
Conventionally, an image sensing apparatus such as a video camera having a function of automatically adjusting exposure in response to a video signal obtained from an image sensor, i.e, a so-called auto iris function has been put into practice. Such a conventional auto iris system mainly includes two types: a system of mechanically adjusting exposure by adjusting an optical iris of a lens and a system of electrically adjusting exposure by adjusting the amplification factor of an amplifier for amplifying a level of a video signal obtained from an image sensor.
However, such a conventional auto iris system has the following disadvantages: When a high luminance portion such as a light source is included in a picture, for example, the size of the aperture of the diaphragm is decreased, so that the gain of the entire picture is decreased, whereby a major object becomes insufficiently bright. On the contrary, when the background is very dark, the size of the aperture of the diaphragm is increased, so that the gain of the entire picture is increased, whereby the major object becomes too bright.
An exposure correcting method for eliminating such disadvantages has been proposed in, for example, Japanese Patent Laying-Open Gazette No. 110369/1987. In this exposure correcting method, in view of the fact that a major object is arranged in the center of the picture in many cases, a picture is divided into a central region and a peripheral region, a signal level of a video signal obtained from an image sensor in each region is detected as an evaluating value, and the evaluating value in the central region is weighted with respect to the evaluating value in the peripheral region (in the extreme case, the central region "1" while the peripheral region "0"). The size of the aperture of the diaphragm and the amplification gain of the video signal are controlled according to the ratio of both the evaluating values, so that contribution of the central region in the picture to the determination of exposure is increased.
Hence, in such an exposure correcting method, an auto iris operation most suitable for the change in brightness of the entire picture is ensured. However, when a high luminance portion is included only in the central region or an object of extremely high luminance such as the sun is included only in the peripheral region, for example, the size of the aperture of the diaphragm is decreased so that the gain of the entire picture is decreased, whereby a major object becomes insufficiently bright.
On the other hand, Japanese Patent Laying-Open Gazette No. 41079/1982, for example, discloses a method in which a picture is divided into a plurality of regions, and a region which is an object of exposure adjustment is manually selected by a user so that exposure adjustment is made in response to only a video signal in the region. However, it is very troublesome for the user to always select such region by manual operation. Furthermore, this method is for selecting an object region not by the objective judgement based on a video signal obtained from an image sensor but by the subjective judgement of the user. Thus, with such a method, it is difficult to perform automatic exposure control with high precision.
Meanwhile, in an image sensing apparatus such as a video camera, it is important to obtain the brightness suitable for an object, i.e., a suitable level of a luminance signal obtained from an image sensor, as described above. Such a level of the luminance signal includes the average brightness in a picture, i.e., an absolute level of the luminance signal obtained from an image sensor and the contrast of the picture, i.e., a relative level thereof.
The former is suitably adjusted by an auto iris operation such as adjustment of an optical iris and control of the amplification gain of a video signal as described above, while the latter is suitably adjusted by a gamma (.gamma.) correction system as described below.
This gamma correction will be described in detail. In general, a dynamic range of the present image sensor (approximately 40 dB) is much smaller than a dynamic range of the luminance of an object (approximately 100 dB), and a dynamic range of a display device such as a CRT (Cathode Ray Tube) is almost the same as or smaller than that of the image sensor (approximately 30 to 40 dB). Thus, even if the brightness of a picture, i.e, an absolute level of a luminance signal is controlled, a high luminance portion in the picture is saturated and a low luminance portion therein is blackened all over, so that it is difficult to obtain the brightness suitable for the entire object.
In an ordinary image sensing apparatus, gamma correction of a luminance signal is made in a circuit on the side of a camera such that overall gamma characteristics of an entire system, including all devices from an image sensor to a display device, based on photoelectric conversion characteristics (gamma characteristics) of the image sensor and nonlinear photoelectric conversion characteristics of the display device is always 1.
However, considering a case in which a high luminance portion such as the sun exists in a picture, for example, resulting in a backlight state, so that the contrast of the picture is significantly increased, if a gamma correction value on the side of a camera remains fixed, the brightness suitable for an object can not be obtained. Therefore, a technique has been proposed in which a plurality of gamma correction circuits respectively having different gamma correction values are provided and the gamma correction value is gradually changed as required.
However, by such a technique, a user of a camera manually operates a switch (so-called backlight switch) by the subjective judgement in the backlight state to select a correction circuit having a small gamma correction value, to maintain brightness most suitable for an object. More specifically, this technique is not automatically used by the objective judgement based on a video signal obtained from an image sensor, whereby it is difficult to perform gamma correction with high precision. In addition, it is also troublesome for the user to always select such suitable gamma correction value by manual operation.
Additionally, the above described conventional auto iris mechanism and gamma correction mechanism are independent of each other. Thus, if an image sensing apparatus is simultaneously equipped with these mechanisms, the image sensing apparatus becomes very complicated from the viewpoint of a mechanism and a circuit.